Fix memory leaks, improve error messages, and clean up technical debt

[DavidLiedle]

This PR addresses several maintenance items I noticed while exploring the codebase. These are mostly small improvements that help with code quality and user experience. They were done with Claude Code.

Changes Made

Bug Fixes

• Fixed a memory leak from setInterval in debug code that wasn't being cleaned up on reset
• Fixed a missing closing quote in an error message string

Code Cleanup

• Removed the deprecated btc command that was just returning an error message
• Removed ~160 lines of commented-out code that had been sitting unused
• Replaced console.log/console.warn statements with proper logger calls

User Experience Improvements

• Standardized generic error messages (replaced "Whoops!", "Oops", etc. with clearer messages)
• Added the missing UI component for git repository rename notifications (the backend already supported this)
• Modernized the browser extension's SPA monitoring from polling to MutationObserver

Performance

• Optimized slice operations in teams/member_set.go to reduce memory allocations

Testing

• Go tests pass for affected packages
• Code compiles without errors
• Changes follow existing patterns in the codebase

Notes

These are mostly small, unglamorous fixes, but I hope they help make the codebase a bit cleaner and more maintainable. Happy to make any adjustments based on your team's preferences or coding standards.